Boron nitride agglomerates, method of production thereof and use thereof

ABSTRACT

The invention relates to boron nitride agglomerates, comprising lamellar, hexagonal boron nitride primary particles, which are agglomerated with one another with a preferred orientation, the agglomerates formed being flake-shaped. 
     The invention also relates to a method for producing said boron nitride agglomerates, characterized in that lamellar, hexagonal boron nitride primary particles are agglomerated in such a way that they line up with one another with a preferred orientation. 
     The flake-shaped agglomerates according to the invention are suitable as filler for polymers for making polymer-boron nitride composites and for hot pressing of boron nitride sintered compacts.

FIELD OF THE INVENTION

The present invention relates to boron nitride agglomerates, comprisinglamellar, hexagonal boron nitride, a method of production thereof andthe use of said agglomerates as filler for polymers and for the hotpressing of boron nitride sintered compacts.

BACKGROUND OF THE INVENTION

Hexagonal boron nitride powder can, owing to its good thermalconductivity, be used as filler for polymers in applicationssimultaneously requiring good electrical insulation capability of thefiller used. Furthermore, boron nitride powder is also used as sinteringpowder for hot pressing, for applications in metallurgy. Moreover,hexagonal boron nitride powder is used in cosmetic preparations, as alubricant, as a parting compound in metallurgy and as raw material forthe production of cubic boron nitride.

Hexagonal boron nitride powder is synthesized industrially by nitridingboric acid in the presence of a source of nitrogen. Ammonia can be usedas the source of nitrogen, and then usually calcium phosphate is used asthe carrier material for the boric acid. An organic source of nitrogensuch as melamine or urea can also be reacted under nitrogen with boricacid or borates. Nitriding is usually carried out at temperatures from800 to 1200° C. The boron nitride then obtained is largely amorphous,and it is also called turbostratic boron nitride. Hexagonal, crystallineboron nitride is produced from amorphous boron nitride at highertemperatures up to about 2100° C. preferably in a nitrogen atmosphere.For this high-temperature treatment, crystallization additives are alsoadded to the amorphous boron nitride.

In the high-temperature treatment, hexagonal boron nitride (hBN) isformed, as primary particles with lamellar morphology. Typical sizes ofthe lamellae are in the range from approx. 1 to 20 μm, but sizes of thelamellae up to 50 μm or more are also possible. Usually the heat-treatedproduct is ground or deagglomerated after production, to obtainprocessable powder.

The thermal conductivity of hexagonal boron nitride is greater in theplane of the lamella (a-axis) than perpendicular to it (c-axis). In thedirection of the c-axis the thermal conductivity is 2.0 W/mK, but in thedirection of the a-axis it is 400 W/mK (see R. F. Hill, SMTA NationalSymposium “Emerging packaging Technologies”, Research Triangle Park,N.C., Nov. 18-21, 1996).

As well as lamellar boron nitride primary particles or agglomerates ofsaid primary particles, which are formed in the synthesis of hexagonalboron nitride, hexagonal boron nitride powder for uses as filler arealso often used in the form of specially produced granules, i.e. in theform of secondary particles formed from the primary particles.Granulation improves the processing properties, such as the free-flowingproperties and metering properties, of the boron nitride powder, andhigher degrees of filling and higher thermal conductivities can beachieved for example in polymer-boron nitride composites. There arevarious methods for production of these secondary particles, givinggranules with varying morphology and varying properties.

The specially produced granules are often also called “agglomerates”,just as for the agglomerates or aggregates produced in the synthesis ofhexagonal boron nitride.

PRIOR ART

Known methods for the production of granules are pelletization and spraygranulation. The starting point in spray granulation is a suspension ofsolid in a liquid, which is atomized to droplets and these are thendried. In pelletization, a small amount of liquid is added to the solid,and because of surface wetting and capillary forces this leads toagglomeration of the solid primary particles, and the agglomerates arethen dried. Binders are usually employed in both methods. Secondaryparticles of low density and/or high porosity are obtained in bothmethods.

US 2006/0 127 422 A1 describes a method for producing spherical boronnitride agglomerates, in which lamellar hexagonal boron nitride isspray-dried from an aqueous suspension with an organic binder.Spray-drying leads to spherical boron nitride granules with an averageagglomerate size from 1 to 500 μm. In contrast to the starting powder,the sprayed granules are flowable.

WO 03/013 845 A1 describes a method for producing spherical boronnitride granules, in which primary particles of hexagonal boron nitride,with addition of polycarboxylic acids, silanes or organometalliccompounds, are spray-dried and the sprayed granules obtained are thensintered at temperatures between 1800 and 2400° C.

A disadvantage in the methods of granulation of boron nitride byspray-drying is that it is necessary to use binders, and the organicbinders used for the spray-drying must be adapted to the particularsystem for further processing as filler for polymers. The granulesobtained in spray-drying are of low density, and when used as filler forthermoplastics the agglomerates may disintegrate completely.

One possibility for making boron nitride granules of high density is thecomminution of hot-pressed boron nitride. This results in granules ofhigh density, corresponding to the density of the hot-pressed compacts.However, a disadvantage in this method is that it is first necessary toproduce hot-pressed boron nitride articles at temperatures of about1800° C. and at an axial compaction pressure of typically 20 MPa fromboron nitride powder (i.e. from boron nitride primary particles), whichis a laborious and expensive process. The subsequent process step ofspecial comminution of the hot-pressed articles is also expensive.

Another method for producing boron nitride granules for use as filler isdescribed in U.S. Pat. No. 6,048,511 and EP 0 939 066 A1. In this,hexagonal boron nitride powder is processed into particles, the sizedistribution of which ranges over a minimum size range of 100 μm, theground hBN powder is compacted cold, then granules are produced from thecold-compacted material by disintegration, and finally the resultantgranules are sieved, to obtain agglomerates with a desired size range.By multiple repetition of the steps of disintegration and cold pressing,material can be compacted with a density of up to 1.91 g/cm³, from whichgranules are produced by disintegration. This method has thedisadvantage that it is very expensive, as it is first necessary toobtain a special size distribution of the starting powder, and thenseveral steps of compaction and comminution are required.

In US 2002/0 006 373 A1, briquettes of agglomerated boron nitridelamellae, which are formed in the production of hexagonal boron nitrideduring high-temperature treatment under nitrogen at 1400 to 2300° C.,are ground, forming a powder, which contains agglomerates of hexagonalboron nitride and non-agglomerated boron nitride platelets, and then thenon-agglomerated platelets are removed, giving a powder consisting ofagglomerates of hexagonal boron nitride platelets with a sizedistribution of the agglomerates from 10 to 125 μm.

US 2004/0 208 812 A1 describes a method for producing a boron nitridepowder containing boron nitride agglomerates, in which hexagonal boronnitride with an average platelet size of at least 2 μm is compacted togreen compacts, the green compacts are then sintered at temperaturesabove 1400° C. up to densities from 1.4 to 1.7 g/cm³ and the sinteredcompacts obtained are then ground.

WO 2005/021 428 A1 describes a method for producing boron nitrideagglomerates of low and medium density, in which turbostratic orhexagonal boron nitride powder with a maximum particle size of 5 μm isheat-treated above 1400° C., preferably at 1850 to 1900° C., and is thenground. Before the thermal treatment, the boron nitride powder can becompressed isostatically into compacts and can be ground.

The resultant agglomerates are spherical to cube-shaped and theagglomerates have isotropic properties, i.e. the primary particles areunoriented in the agglomerates.

U.S. Pat. No. 5,854,155 and U.S. Pat. No. 6,096,671 describe a methodfor producing aggregated lamellar boron nitride particles, in which theboron nitride lamellae are unoriented in the aggregates, i.e. they donot have a preferred direction, and they are bound together withoutbinder. The boron nitride aggregates are pinecone-shaped, and arealready formed during synthesis of the hexagonal boron nitride fromboric acid, melamine and crystallization catalyst. These aggregates areof very low density and they only have slight mechanical stability.

A common feature of the methods described so far for producing boronnitride granules is that they all consist of unoriented lamellar boronnitride particles. For use as a filler for increasing the thermalconductivity of polymers, the lamellar shape of hexagonal boron nitrideis a disadvantage (as described above for example in WO 03/013845 A1 onpage 2). The small primary particle size and the often associated highspecific surface of the hexagonal boron nitride powder limit thepossibilities for processing and application. The processing propertiescan be improved by producing isotropic agglomerates with unoriented hBNlamellae.

When using boron nitride powder as filler for polymers, the aim is toachieve the highest possible thermal conductivity for the polymer-boronnitride composites, so that any heat produced can be removed as well aspossible. For a particular value to be achieved for the thermalconductivity of the polymer-boron nitride composite, the degree offilling, and thus the amount of boron nitride to be used, should be assmall as possible.

If now the boron nitride primary particles in the polymer-boron nitridecomposite are oriented isotropically, i.e. they have no preferredorientation, the good thermal conductivity of boron nitride in the planeof the lamella (“in-plane”) cannot be fully utilized, as heat conductionin such a material is admittedly often in the plane of the lamella, butit also often takes place perpendicularly to the plane of the lamella ofhexagonal boron nitride. Now if isotropic boron nitride agglomerates areused for making said composites, in which the primary particles in theagglomerates are not oriented, the distribution of the boron nitrideprimary particles in the polymer-boron nitride composites is alsoisotropic and the good thermal conductivity of boron nitride in theplane of the lamella cannot be utilized optimally.

As well as the production of granules, other ways have been describedfor increasing the thermal conductivity of boron nitride-filledpolymers.

For example, US 2003/0 153 665 A1 describes a method in which a magneticfield is applied to a polymer blend containing hexagonal boron nitridepowder, as a result of which the boron nitride lamellae of the hexagonalboron nitride powder are oriented in a particular direction, and thenthe polymer blend with the oriented boron nitride powder is cured. Inthis way it is possible to obtain filled polymers with increased thermalconductivity in the direction of orientation of the boron nitridelamellae. There is the disadvantage, however, that an external magneticfield is required for orientation of the boron nitride lamellae.

In WO 2008/085 999 A1, a polymer material filled with boron nitride isproduced, in which hexagonal boron nitride is mixed with a polymer andis cured with introduction of force, for example by extrusion, so thatthe boron nitride lamellae are aligned. In the example, severalspecimens obtained in this way with a thickness of 1 mm are stacked ontop of one another and hot-pressed at 130° C. Disks are machined fromthe resultant compact, perpendicularly to the direction of orientationof the boron nitride lamellae. The through-plane thermal conductivitymeasured on the resultant disks is higher than that of the compactproduced originally. However, the specimens must always be removedperpendicularly to the direction of orientation of the boron nitridelamellae, so that the method is expensive and cannot be used for allapplications. For example, the method is only suitable forthermoplastics, but not for thermosets or pastes.

THE TASK OF THE INVENTION

The invention is therefore based on the task of overcoming the drawbacksof the prior art and making available boron nitride agglomerates of highdensity, with which the anisotropic properties of hexagonal boronnitride can be better utilized, in particular for applications as fillerfor polymers.

Furthermore, the invention is based on the task of making available acost-effective, simple method for producing boron nitride agglomeratesof high density.

SUMMARY OF THE INVENTION

The above task is achieved according to the invention with boron nitrideagglomerates according to claim 1, a method for producing said boronnitride agglomerates according to claim 11 and a polymer-boron nitridecomposite according to claim 17 and a boron nitride sintered compactaccording to claim 18.

The invention therefore relates to boron nitride agglomerates,comprising lamellar, hexagonal boron nitride primary particles, whichare agglomerated together with a preferred orientation, the agglomeratesbeing flake-shaped.

The invention also relates to a method for producing said boron nitrideagglomerates, in which lamellar, hexagonal boron nitride primaryparticles are agglomerated in such a way that they line up with oneanother with a preferred orientation.

The invention further relates to a polymer-boron nitride compositecomprising flake-shaped boron nitride agglomerates according to theinvention and a boron nitride sintered compact, obtainable by sinteringflake-shaped boron nitride agglomerates according to the invention.

In contrast to the known agglomerates, for which the boron nitrideprimary particles in the agglomerates are agglomerated with one anotherlargely without preferred orientation, the primary particles in theagglomerates according to the invention have a definite preferredorientation—they are oriented with one another and agglomerated. Theboron nitride agglomerates according to the invention can, based on thepreferred direction of the boron nitride primary particles, also bedesignated as textured boron nitride agglomerates.

The boron nitride agglomerates according to the invention areflake-shaped and can, on the basis of their flake shape, also be called“flakes”, and the flakes according to the invention are to bedifferentiated from non-agglomerated lamellar boron nitride primaryparticles, which are often also called “flaky boron nitrides particles”in the literature in English. The structure of the agglomeratesaccording to the invention is made up of many individual boron nitridelamellae.

In contrast to non-agglomerated boron nitride powders, the agglomeratesaccording to the invention are free-flowing and can be metered easily.

The textured agglomerates according to the invention have a highdensity, which can even be higher than that of the high-densityagglomerates in EP 0 939 066 A1.

Moreover, the agglomerates according to the invention have goodmechanical stability, and the mechanical stability can be furtherimproved markedly by thermal treatment of the agglomerates.

Owing to the preferred orientation of the boron nitride primaryparticles in the agglomerates, in contrast to the known agglomerates,the agglomerates according to the invention have anisotropic properties.In particular, the thermal conductivity is higher in the direction ofthe diameter of the flakes than in the direction of the thickness of theflakes, because in the direction of the diameter of the flakes, i.e. inthe plane of the flake-shaped agglomerates, heat conduction largelytakes place in the plane of the lamellae of the boron nitride primaryparticles, in which the thermal conductivity is higher.

Surprisingly, it was found that agglomerates of high density and withgood mechanical stability can be produced by the method according to theinvention.

It is also surprising that highly textured boron nitride agglomeratescan be produced, in which the boron nitride primary particles are verystrongly oriented to one another, in contrast to the known isotropicgranules, in which there is hardly any or at best only very slighttexturing. The texturing is also markedly greater than in hot-pressedboron nitride.

Compared with the methods known in the prior art for producing boronnitride granules, the method according to the invention has theadvantage that it is less expensive, that granules of high density canbe produced in just one step and that it is suitable as a continuousprocess for producing large amounts of boron nitride agglomerates.

Surprisingly, it was also found that the textured boron nitrideagglomerates according to the invention can also be used as filler forpolymers and high thermal conductivity values are then obtained. This issurprising because up to now the lamellar particle morphology of theprimary particles, combined with the anisotropy of the thermalconductivity of the primary particles, was regarded as unfavourable forapplications as filler, and this disadvantage could only be overcome bythe production and use of isotropic granules.

Owing to the anisotropic properties of the agglomerates according to theinvention, the anisotropic thermal conductivity properties of hexagonalboron nitride can be better utilized. Polymers filled with isotropicboron nitride agglomerates having a spherical or cube shape also haveisotropic thermal conductivity properties. When using the textured boronnitride agglomerates according to the invention with anisotropicproperties, the anisotropy of the thermal conductivity of the polymer-BNcomposite can be adjusted. If the textured agglomerates with a lamellarshape are used, then for example in injection moulding of thecorresponding polymer-boron nitride compounds, say of plates or ribs, onaccount of wall friction, polymer-boron nitride composites areinevitably obtained with a preferred orientation of the textured boronnitride agglomerates. Polymer-boron nitride composites produced in thisway have, with orientation of the agglomerates in the preferreddirection, higher thermal conductivity values in the preferred direction(“in-plane” relative to the plane of the lamellae) than perpendicularlyto the preferred direction (“through-plane”). Surprisingly, with thesepolymer-boron nitride composites it is not only possible to obtain highthermal conductivity values in-plane, but also high thermal conductivityvalues through-plane.

In contrast to some methods known in the prior art for producing boronnitride granules or agglomerates, with the method according to theinvention it is possible to produce binder-free agglomerates.

BRIEF DESCRIPTION OF THE APPENDED DRAWINGS

FIG. 1 a shows an SEM micrograph of the agglomerates according to theinvention with smooth forming surfaces and rough fracture surfaces.

FIG. 1 b shows a schematic drawing of the SEM micrograph in FIG. 1 a,with smooth forming surfaces 1 and rough fracture surfaces 2.

FIG. 2 shows schematically the production of agglomerates according tothe invention by compacting between two counterrotating rolls 3, 4arranged without a gap, where one of the rolls is driven.

FIG. 3 shows schematically the powder fill 5 above the two rolls 3, 4.The particles to be agglomerated become oriented in a narrow regionabove the “roll gap”.

FIG. 4 shows schematically the filled roll gap 6. In the gap 6, thepre-oriented particles become fully oriented and, under the highpressure in the roll gap, are compacted to textured agglomerates of highdensity. At an agglomerate thickness of for example 50 μm, the texturedagglomerates have forced the two rolls apart by 50 μm.

FIG. 5 shows, with a schematic cross-section, the structure of thetextured agglomerates 7 according to the invention. Primary particles 8of hexagonal boron nitride are densely packed and, moreover, are largelyoriented parallel to one another, i.e. they have a preferred orientationto one another. The roughness of the forming surface (top and bottom inthe schematic drawing) is in the region of the thickness of the primaryparticles (<1 μm). The roughness of the surfaces of the lateral regions,i.e. the fracture surfaces or edge zones produced during compaction, isin the region of the average primary particle diameters.

FIG. 6 shows, with a schematic representation, how suitable comminution,e.g. cutting or breaking, of textured boron nitride agglomeratesperpendicularly to the forming surface can again produce texturedagglomerates, with the orientation of the boron nitride primaryparticles in the resultant textured agglomerates being reversed orrotated: the orientation of the primary particles in the newly producedagglomerates is no longer parallel, but perpendicular to the mainsurface of the agglomerates. The textured agglomerates produced in thisway can be oriented in polymer-boron nitride composites in the formingprocess, so that increased through-plane thermal conductivity can beobtained.

DETAILED DESCRIPTION OF THE INVENTION

As already mentioned above, the boron nitride agglomerates according tothe invention are agglomerates of lamellar, hexagonal boron nitrideprimary particles, which have been agglomerated together with apreferred orientation and therefore can also be called oriented ortextured agglomerates or granules.

Preferably, the boron nitride primary particles are agglomerated with apreferred orientation in such a way that the planes of the lamellae ofthe boron nitride primary particles are essentially aligned parallel toone another, i.e. so that the majority of the boron nitride primaryparticles are oriented parallel or almost parallel to one another.

The degree of orientation of the lamellar boron nitride primaryparticles in the agglomerates according to the invention can becharacterized by the texture index. The texture index of hexagonal boronnitride with isotropic orientation of the lamellar boron nitride primaryparticles, thus without preferred orientation, has a value of 1. Thetexture index increases with the degree of orientation in the sample.The texture index of the agglomerates according to the invention hasvalues above 1.5, preferably 2.0 or more, more preferably 2.8 or more,and especially preferably 3.5 or more. The texture index of theagglomerates according to the invention can also have values of 5.0 ormore and of 10.0 or more.

The texture index is determined by an X-ray method. For this, the ratioof the intensities of the (002) and of the (100) reflection measured onX-ray diffraction diagrams is determined and is divided by thecorresponding ratio for an ideal, untextured hBN sample. This idealratio can be determined from the JCPDS data and is 7.29.

The texture index (TI) can therefore be determined from the formula

${TI} = {\frac{I_{{(002)},{sample}}/I_{{(100)},{sample}}}{I_{{(002)},{theoretical}}/I_{{(100)},{theoretical}}} = {\frac{I_{{(002)},{sample}}/I_{{(100)},{sample}}}{7.29}.}}$

For the agglomerates according to the invention with a size of about 3.5cm² (relative to the area of the top or underside of the flake-shapedagglomerates), very high values of 100 or more can be obtained for thetexture index. These values measured on the large flake-shapedagglomerates are evidence of the very marked orientation of the primaryparticles in the agglomerates according to the invention. The textureindex for smaller agglomerates, say smaller than 1 mm, is measured onbeds of agglomerates. There is then partially random orientation in thesample carrier for the X-ray measurement. Therefore the values obtainedon smaller textured agglomerates for the texture index are always lowerthan corresponds to the orientation of the primary particles in theindividual flake-shaped agglomerate.

The agglomerate size of the agglomerates according to the invention canbe stated as the sieving fraction, for example as “<350 μm” or “100-200μm”, or as the average agglomerate size (d₅₀) determined by measurementof the agglomerate size distribution. The agglomerate size distributioncan be measured by laser diffraction (dry measurement, Mastersizer 2000,Malvern, Germany).

The agglomerates according to the invention can have agglomerate sizesof several centimetres. For further processing or use, agglomerate sizesup to about 1 cm are advisable, depending on the particular intendedapplication. For use as filler for polymers, usually the most variedmaterials are used with agglomerate sizes up to 3 mm, preferably up to 1mm. More preferably, average agglomerate sizes (d₅₀) of up to 500 μm,and more preferably of up to 200 μm are used. The d₅₀ value ispreferably at least 30 μm. Especially preferably, narrow grain sizeranges are used, for example 100 to 300 μm, 1 to 2 mm or 50 to 150 μm.

The flake-shaped agglomerates according to the invention can have athickness from 10 to 500 μm, preferably from 15 to 350 μm, especiallypreferably from 30 to 200 μm.

The aspect ratio, i.e. the ratio of agglomerate diameter to agglomeratethickness, of the flake-shaped agglomerates can be determined based onSEM photographs, by measuring the agglomerate diameter and thickness.

The aspect ratio of the flake-shaped agglomerates according to theinvention has a value greater than 1, preferably values of 1.5 or more,more preferably values of 2 or more.

The density of the agglomerates according to the invention is preferably1.6 g/cm³ or more, preferably 1.8 g/cm³ or more and especiallypreferably 2.0 g/cm³ or more.

The density of the agglomerates according to the invention can bedetermined on agglomerates with a size of about 1-5 cm² according to theArchimedes principle as the buoyancy density in water or geometricallyon agglomerates produced by cutting with a size of about 1 cm×1 cm.

For use as filler for polymers it is advantageous to use fillers oragglomerates with the lowest possible specific surface (BET), as thisminimizes the heat transmission resistances from the filler to thepolymer. With the method according to the invention it is possible toproduce boron nitride agglomerates with very small specific surface,much smaller than the agglomerates of the prior art.

The textured agglomerates according to the invention have surfaces ontheir top and underside that are produced directly by the formingprocess, not by comminution. These surfaces, designated as “formingsurfaces”, are comparatively smooth, in contrast to the rough lateralsurfaces (fracture surfaces) of the agglomerates, which were produced bybreaking or by comminution operations. The surfaces of the flake-shapedagglomerates according to the invention are essentially flat (planar)and their top surface and underside are largely parallel to each other.

The proportion of the forming surface in the total surface of theagglomerates according to the invention is, assuming a lamellar or flakeshape with round main surface, on average at least 33% (if the diameterof the agglomerates is equal to their height) and, assuming a lamellaror flake shape with square main surface, also at least 33% (for the cubeshape). For agglomerates according to the invention with high aspectratio the proportion of the forming surfaces in the total surface ismuch higher; for agglomerates with an aspect ratio>3.0 the proportion isusually between 60 and 95%, and for very large agglomerates theproportion can be even higher. By rounding the agglomerates or as aresult of a sieving or classification process, the proportion of theforming surfaces in the total surface may be reduced, but as a rule theproportion is always at least 10%, preferably at least 20%.

The ratio of forming surface to total surface can be determined byevaluation of SEM photographs. The values found for agglomerate diameterand thickness for determining the aspect ratio are used for this. Theproportion of the forming surface in the total surface is determinedfrom these values as follows:

Proportion of forming surface [%]=2*front area/total surface*100

where

front area=agglomerate diameter*agglomerate diameter

total surface=2*front area+4*side area side area=agglomeratethickness*agglomerate diameter

For production of the textured agglomerates according to the invention,preferably boron nitride powder of hBN is compacted between twocounterrotating rolls to form textured agglomerates.

For this, boron nitride powder or the starting material containing boronnitride powder used for agglomeration is fed continuously in a uniformamount into the space between two counterrotating rolls. Because therolls rotate in opposite directions, the shearing of the agglomerates isminimized. The gap, i.e. the distance between the rolls, is preferablyat most 500 μm. In a preferred embodiment the rolls are pressed togetherwith a specified contact pressure. There is then no specified gapbetween the two rolls, rather the rolls are in contact. The supported oradhering boron nitride powder is entrained and strongly compactedbetween the rolls. If no gap is set between the two rolls, duringcompaction the rolls are forced apart by the textured agglomerates, andthe resultant gap corresponds to the thickness of the agglomerates. Thematerial used for making the rolls should have the maximum possiblehardness, so that the roll gap geometry remains unchanged. The rolls canbe made of a ceramic material, for example silicon nitride.

During metering of the boron nitride powder into the space between thetwo rolls, it is necessary to ensure that sufficient boron nitridepowder goes into the space between the two rolls, so that the particlesin the roll gap come in contact and are compacted. With insufficientfeed there is little or no compaction and no, or hardly any,agglomerates form. When metering is sufficient, there is a maximumfilling volume above the roll gap, depending on the roll geometry,rolling parameters and starting powder. If metering is excessive, theregion above the roll gap may “overflow”. If the starting material usedis too coarse, the roll gap may become clogged, with locking of therolls.

It is also possible to provide the rolls with surface structuring. Forexample, surface structuring can be used in order to produce requiredbreaking points in the flake-shaped agglomerates, which determine thesize and shape of the agglomerates. The compacted agglomerates can thuseasily be comminuted to agglomerates of a specified size and shape. Thestructuring of the rolls can for example produce a planar-polygonalagglomerate shape.

The boron nitride powder used can be hexagonal boron nitride andmixtures of hexagonal boron nitride with amorphous boron nitride, aswell as partially crystalline boron nitride.

The average particle size d₅₀ of the hexagonal boron nitride powder usedcan be 0.5-50 μm, preferably 0.5-15 μm. For example, it is possible touse hexagonal boron nitride powders with an average particle size of 1μm, 3 μm, 6 μm, 9 μm and 15 μm, but larger average particle sizes up to50 μm are also possible. Mixtures of various hexagonal boron nitridepowders with different particle sizes can also be used. The averageparticle size (d₅₀) of the boron nitride powder used is usually measuredby laser diffraction (wet measurement, Mastersizer 2000, Malvern).

It is possible to use B₂O₃-free boron nitride powder and boron nitridepowder with low B₂O₃ contents of up to 0.5 wt. %, but also with higherB₂O₃ contents of up to 10 wt. % or more.

It is also possible to use boron nitride granules, for examplepelletized granules and sprayed granules of hexagonal boron nitride, oralso agglomerates that formed during synthesis of the hexagonal boronnitride.

The boron nitride used for the roll compacting can also besurface-coated with additives, the latter preferably being selected fromthe group comprising polymers, organometallic compounds, silanes, oils,carboxylic acids, copolymers, hydrolysed monomers, partially hydrolysedmonomers, partially condensed monomers and nanoparticles.

It is also possible to use mixtures of pulverulent or granulated boronnitride and other powders and therefore produce mixed agglomerates(“hybrid flakes”). Other particles can be used, selected from the groupcomprising carbides, borides, nitrides, oxides, hydroxides, carbon andmetals. These substances can be used both as particles, and as fibres orlamellae of any fineness. Furthermore, polymers can also be processedtogether with boron nitride into textured agglomerates. For example,SiO₂ powder, preferably nanoscale SiO₂ powder such as finely dividedsilica, or also aluminium oxide, boehmite or Al(OH)₃, preferably asnanoscale powder, or also aluminium nitride, can be added to the boronnitride powder. If metal particles are added, these can be nitrided in athermal treatment under nitrogen following compaction. Furthermore,polymers and precursors of ceramic substances, for example salts oralkoxides, can be added. Mixtures of said additives can also be usedtogether with hexagonal boron nitride for the production of mixedgranules, and mixed granules from these powder mixtures can also be usedfor compaction. It is also possible to use the additives in the form ofgranules and use them in a mixture together with pulverulent orgranulated boron nitride as starting material for compaction.

For compacting, it is also possible to use sprayed and pelletizedgranules of hexagonal boron nitride powders with sol-gel binders such asfor example boehmite, MTEOS (methyltriethoxysilane), sodium silicate,silica sol and mixtures with nanoparticles, or also sol-gel-coated boronnitride powder, and sol-gel systems such as for example boehmite, MTEOS(methyltriethoxysilane), sodium silicate, silica sol and mixtures withnanoparticles can be used for coating.

Depending on the nature of the powder materials, in particular theirinitial particle size and size distribution, passage through the rollsresults in flakes of varying size with varying diameter and uniformthickness, and as a rule fractions of uncompacted material are alsopresent.

The compacting operation can be repeated several times. By compactingtwo or three times, the uncompacted fines fraction can be reduced fromup to 50 wt. % to below 5 wt. %. An increase in size of the agglomerateswill be observed, both with respect to the diameter and their thickness.

Between two successive compaction steps, the resultant uncompacted finesfraction can be separated by sieving and can be returned to thepreceding step.

The fines fraction can, however, also be left with the compacted productrather than being separated. In this case the resultant fines fractionis minimized in a subsequent compression step, as it too is compacted toagglomerates.

When using a fine starting material with an average particle size (d₅₀)of approx. 1 μm, for example agglomerates with a grain size range of 1-4mm can be obtained in a first compaction. In a second compaction theagglomerate diameter can increase to approx. 1 cm. In a finalcompaction, textured agglomerates up to 2 cm in size can be obtained.

When using a coarser starting material of primary particles with anaverage particle size (d₅₀) of 15 μm, for example in the first passtextured agglomerates with a size of 1-2 cm can be obtained, in thesecond pass textured agglomerates of 3-4 cm, and in the last compactionstep 5-7 cm is possible.

If the starting material used is not powder, but pelletized granules,sprayed granules or the like, then from granules with an average granulesize of for example 100-200 μm, textured agglomerates with a size ofseveral centimetres are produced in the first pass, and “infinite”agglomerates are obtained in the second pass.

Compaction is optionally preceded, depending on the starting material tobe used, advantageously by a protective sieving or classification, toprevent damage or blocking of the rolls.

The material compacted to textured agglomerates can undergo a thermaltreatment step, sintering. This thermal treatment of the agglomeratescan further improve the mechanical stability and therefore also theprocessability, for example the metering properties.

The thermal conductivity achievable in boron nitride polymer compositescan also be further increased by sintering of the flake-shapedagglomerates.

The thermal treatment may result in changes in the properties of theagglomerates and primary particles. The degree of crystallization of theprimary particles may increase, which is associated with growth of theprimary particles. The lattice oxygen content and the specific surfacedecrease with increasing temperature of thermal treatment and durationof thermal treatment. Density, hardness and mechanical stability can bemaximized depending on the temperature of thermal treatment and theduration of thermal treatment.

Depending on the sintering conditions and the nature of the texturedagglomerates, loose, individual agglomerates with a ceramic clinkingsound are obtained, or a sinter cake composed of textured agglomerates,which can be broken up for example by shaking or screening and separatedinto loose agglomerates.

The thermal treatment can be carried out at a temperature of up to 2300°C., preferably in the temperature range 1200-2050° C., more preferably1400-2000° C. and especially preferably 1600-1950° C. in a protectivegas atmosphere. In the case of mixed agglomerates (hybrid flakes) thethermal treatment can be carried out for example at temperatures up to2000° C. in a protective gas atmosphere or in air or also with areactive gas, for example ammonia or carbon monoxide or gas mixtures.After the maximum temperature is reached, a holding time of up toseveral hours or days can be applied. Nitrogen or argon can be used asthe protective gas. The thermal treatment can be carried out in acontinuous process or in a batch process.

After compaction or optionally after subsequent thermal treatment, theagglomerates or the sinter cake obtained can be processed to the desiredtarget agglomerate size.

The target size for the agglomerate size depends mainly on theparticular use. For use as filler for polymers, it depends for exampleon the proposed processing technology and the desired degree of filling,and the properties of the particular plastic and processing variablessuch as the viscosity must be taken into account, and it can be adaptedto the respective application conditions, for example use as filler forthermoplastics, as filler for thermosets, processing by injectionmoulding, extrusion or cast resin and film extrusion.

The target agglomerate size can be achieved by the usual steps such assieving, sieve-breaking and classifying. Any fines fraction present canbe removed first. The agglomerates with a size of several millimetres toseveral centimetres are processed in a further process step to definedagglomerate sizes. For this it is possible for example to usecommercially available sieves of different sieve mesh and sieving aidson a vibrating sieve. A multistage sieving/sieving-comminution processhas proved advantageous.

To achieve the target agglomerate size, after compaction and/or afterthe thermal treatment and optional removal of the fines fraction, theagglomerates are first preferably passed through a sieve with a meshsize of 3-4 mm. Simple sieving aids, such as rubber balls or plasticrings, are sufficient to break the agglomerates, which typically have athickness of approx. 30 to 350 μm, by a jolting motion. Below a sievemesh of 1 mm, for further sieving comminution it is possible to usesteel balls, preferably rubber-coated steel balls, to obtain agglomeratefractions of for example <700 μm, <500 μm or <350 μm.

In the fractionation steps, finer agglomerate fractions are produced toa varying extent. For example fractions <200 μm, <150 μm and <100 μm canbe isolated in decreasing amount. In order to obtain finer agglomeratesizes below 350 μm with a narrow agglomerate size distribution, thesecan in their turn be produced from the fraction <350 μm obtained asdescribed above, by sieving or also by classifying. In this way, forexample fractions of 100-200 μm, 80-150 μm and 150-200 μm can beobtained. Agglomerate fractions below 100 μm are preferably produced byclassifying, for better adherence to the upper and lower limits of theagglomerate sizes.

As in alternative to sieving, the specified comminution of texturedagglomerates can also take place by sieving-trituration, in classifiermills, structured roll crushers and cutting wheels. Dry grinding forexample in a ball mill is also possible.

For using the textured agglomerates for particular applications, it isadvantageous to submit the agglomerate size fractions obtained to afurther, specific treatment. Examples of such treatments arewet-chemical preparation, for surface removal of adhering B₂O₃, surfacemodification by means of additives, coating by means of fluidized-bedprocesses and thermal/oxidative activation. Surface modification withadditives can for example take place by adsorption from a suspension.Examples of coatings that can be used are polymers, organometalliccompounds, silanes, oils, carboxylic acids, copolymers, hydrolysed,partially hydrolysed and partially condensed monomers, nanoparticlessuch as for example SiO₂ nanoparticles and surface-functionalizednanoparticles, for example surface-functionalized SiO₂ nanoparticles.The additives used for the surface modification can serve as wettingagents or as adhesion promoters.

In another embodiment of the invention, by comminution of largeflake-shaped agglomerates with a thickness of for example 100 or 200 μm,using a multi-blade device, with blade spacing far smaller than thethickness of the agglomerates, for example 50 μm, or by means of avibrated blade, needles and flake-shaped agglomerates can be produced,which are then, as described previously, comminuted by a multistagesieving process and separated into agglomerate fractions. Texturedagglomerates produced in this way display a reversed preferredorientation of the boron nitride primary particles, with their basalplanes perpendicular to the main surface of the flake-shapedagglomerate. Determination of the texture index on textured agglomeratesproduced in this way gives values of 0.7 or less, preferably of 0.5 orless, more preferably of 0.35 or less, especially preferably of 0.3 orless. As a result of this orientation, in filled polymers higherthrough-plane than in-plane thermal conductivity values can be achieved,because the flake-shaped agglomerates produced in this way can line upin the filled polymer, in particular when the aspect ratio of theagglomerates is >2.0. Orientation can take place for example duringcasting of thin plates or in injection moulding.

After compaction or after optional subsequent thermal treatment, thetextured agglomerates according to the invention can undergo mechanicalprocessing. The mechanical processing can take place for example on theroller horse in the plastic container without balls (with a degree offilling of 30 vol. % for one to two hours) or also in the tumble mixer.As a result of this treatment, corners and edges of the texturedagglomerates are rounded and higher solids contents and thermalconductivity values can be achieved in polymer-boron nitride composites.

Apart from the method described for compaction between two rolls, othermethods can also be used for producing the textured agglomeratesaccording to the invention.

One alternative consists of applying a layer of powder on a carrierfilm, for example by spraying with a boron nitride suspension. Withcontrolled evaporation of the suspending medium, a texture is alreadyformed before compaction. By subsequent combined compaction of thecarrier film and the layer of powder, the texture can be furtherenhanced. The carrier film can then be removed, or it can be decomposedin a subsequent thermal treatment step. The resultant compacted layer ofpowder can be processed by breaking and sieving to texturedagglomerates, as described above.

Another possibility comprises doctor-blade film casting, the filmobtained is then compressed or rolled, depending on the organic bindersused, compacted by means of heated rolls and comminuted by cutting totextured agglomerates. Optionally, thermal treatment of the agglomeratescan take place before or after cutting.

The agglomerates according to the invention can be used as filler forpolymers and can be processed to polymer-boron nitride composites. Forproduction of the filled polymers it is also possible to use mixtures ofthe agglomerates according to the invention with other known fillers forpolymers, for example aluminium oxide. For production of the filledpolymers it is also possible to use mixtures of different fractions ofthe agglomerates according to the invention, as well as mixtures of saidfractions with primary particle fractions.

The agglomerates according to the invention, both pure boron nitrideagglomerates and mixed granules (“hybrid flakes”), are suitable asstarting material for the production of boron nitride sinter materialsand boron nitride mixed ceramics. Preferably the boron nitride sintermaterials and boron nitride mixed ceramics are produced by hot pressingor hot isostatic pressing. The agglomerates used for this are preferablyuncalcined agglomerates and agglomerates heat-treated at lowtemperatures up to about 1600° C., for example agglomerates that havebeen treated to remove binders or calcined. The high density and highbulk density of the agglomerates according to the invention ensure muchhigher degrees of mould filling and green densities and therefore moreefficient compaction, than was possible with conventional cold-isostaticcompressed granules, pelletized or sprayed granules. When theagglomerates according to the invention are used as starting materialfor hot pressing, hot-pressed boron nitride sintered compacts with verypronounced texture and very pronounced anisotropic properties can beobtained. Furthermore, textured agglomerates can be mixed withnon-agglomerated boron nitride powders, to serve as reinforcing elementsor nuclei for the crystallization of hexagonal boron nitride during hotpressing. Mixing of heat-treated textured agglomerates with turbostraticboron nitride is particularly advantageous. By using texturedagglomerates, the properties of boron nitride sintered compacts can beimproved, for example an increase in mechanical strength. It is alsopossible to produce boron nitride mixed ceramics, for example based onboron nitride and silicon nitride, based on boron nitride and zirconiumdioxide and based on boron nitride, zirconium dioxide and siliconcarbide, with textured agglomerates according to the invention. Hotpressing takes place in graphite dies at temperatures from about 1600 to2000° C. and at a pressure up to about MPa. The texture index measuredon the sintered compact, for hot-pressed boron nitride sintered compactsproduced with agglomerates according to the invention, can have valuesup to 120 or more. The texture index is measured on samples that areprepared perpendicularly to the direction of hot pressing. Preferablythe texture index measured on the sintered compact has values of atleast 2, more preferably values of at least 3, more preferably values ofat least 5, more preferably values of at least 10 and especiallypreferably values of 20 or more.

The agglomerates according to the invention can also be used for otherapplications, for example as raw material for the synthesis of cubicboron nitride, and as loose fill for heating cartridges.

EXAMPLES

The following examples serve for further explanation of the invention.

Example 1

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μm(measured using Mastersizer 2000, Malvern, wet measurement; from ESKCeramics GmbH & Co. KG, BORONID S1) is fed continuously by means of avibratory chute between two silicon nitride rolls arranged without agap. The roll width is 150 mm, roll diameter 13 cm. The rolls arerotated at 15 rev/min and are squeezed together with a force of 1.7 t.In this way the boron nitride powder is roll-compacted with a throughputof 4 kg/h.

The resultant material is granulated material in the form of flakes witha thickness of about 50 μm and a fines fraction of uncompacted startingmaterial. The material obtained is then heat-treated at 1200° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 55 wt. % (relative tothe total amount of the heat-treated agglomerates).

Example 2

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is granulated material in the form of flakes witha thickness of about 50 μm and a fines fraction of uncompacted startingmaterial and it is then dedusted by sieving to <100 μm, with 46 wt. % ofthe material obtained in roll compaction being separated as finesfraction. The separated coarse fraction is then heat-treated at 1200° C.for 2 hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 5 wt. %.

Example 3

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is granulated material in the form of flakes witha thickness of about 50 μm and a fines fraction of uncompacted startingmaterial and it is then dedusted by sieving to <100 μm, with 46 wt. % ofthe material obtained in roll compaction being separated as finesfraction. The separated coarse fraction is then heat-treated at 1600° C.for 2 hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 4 wt. %.

Example 4

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is granulated material in the form of flakes witha thickness of about 50 μm and a fines fraction of uncompacted startingmaterial and it is then dedusted by sieving to <100 μm, with 46 wt. % ofthe material obtained in roll compaction being separated as finesfraction. The separated coarse fraction is then heat-treated at 1950° C.for 2 hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 3 wt. %.

Example 5

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 5.8 kg/h. The resultant material is granulated material inthe form of flakes with a thickness of about 50 to 100 μm and a finesfraction of uncompacted starting material and it is then dedusted bysieving to <100 μm, with 26 wt. % of the material obtained in rollcompaction being separated as fines fraction.

The separated coarse fraction is then heat-treated at 1950° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 3 wt. %.

Example 6

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 5.8 kg/h and for a third time with a throughput of 7 kg/h.After each pass through the rolls, the granulated material obtained isdedusted by sieving to <100 μm, finally with 5 wt. % of the materialobtained in roll compaction being separated as fines fraction. Thethickness of the agglomerates obtained in the form of flakes is about 50to 200 μm.

The separated coarse fraction is then heat-treated at 1950° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 4 wt. %.

Example 7 I)

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 5 kg/h and for a third time with a throughput of 6.5 kg/h.Between each compacting operation the fines fraction (<100 μm) wasdetermined, but not separated. The resultant material is granulatedmaterial in the form of flakes with a thickness of about 50 to 200 μmand it is then dedusted by sieving to <100 μm, with 15 wt. % of thematerial obtained in roll compaction being separated as fines fraction.

The separated coarse fraction is then heat-treated at 1200° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving.

Example 7 I) A)

A <100 μm fraction is prepared from example 7 I) by sieving.

Example 7 I) B)

A 100-210 μm fraction is prepared from example 7 I) by sieving.

Example 7 I) C)

A 210-350 μm fraction is prepared from example 7 I) by sieving.

For examples 7 I) A, B) and C), the values determined on theseagglomerate fractions for the texture index and the specific surfaceaccording to the BET method (nitrogen adsorption, Beckmann-Coulter) arepresented in Table 1, and Table 1 also shows, for examples 7 I) A) andB), the aspect ratio determined from SEM photographs.

Example 7 II)

Example 7 I) was repeated, but the coarse fraction separated after rollcompaction three times was heat-treated at 1600° C. for 2 hours in anitrogen atmosphere.

Example 7 II) A)

A <100 μm fraction is prepared from example 7 II) by sieving.

Example 7 II) B)

A 100-210 μm fraction is prepared from example 7 II) by sieving.

Example 7 II) C)

A 210-350 μm fraction is prepared from example 7 II) by sieving.

For examples 7 II) B) and C), the values determined on these agglomeratefractions for the texture index, the specific surface according to theBET method (nitrogen adsorption, Beckmann-Coulter) and the aspect ratiodetermined from SEM photographs are presented in Table 1.

Example 7 III)

Example 7 I) was repeated, but the coarse fraction separated after rollcompaction three times was heat-treated at 1950° C. for 2 hours in anitrogen atmosphere.

Example 7 III) A)

A <100 μm fraction is prepared from example 7 III) by sieving.

The average agglomerate size (d₅₀) is 78 μm.

Example 7 III) B)

A 100-210 μm fraction is prepared from example 7 III) by sieving.

The average agglomerate size (d₅₀) is 156 μm.

Example 7 III) C)

A 210-350 μm fraction is prepared from example 7 III) by sieving.

The average agglomerate size (d₅₀) is 347 μm.

For examples 7 III) B) and C), the values determined on theseagglomerate fractions for the texture index, the specific surfaceaccording to the BET method (nitrogen adsorption, Beckmann-Coulter) andthe aspect ratio determined from SEM photographs are presented in Table1.

Example 8

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μm(measured using Mastersizer 2000, Malvern, wet measurement; from ESKCeramics GmbH & Co. KG, BORONID S15) is fed continuously by means of avibratory chute and is roll-compacted with a throughput of 5.2 kg/h asin example 1.

The granulated material obtained in the form of flakes with a thicknessof about 50 μm contains 28 wt. %<100 μm. Then, without first separatingthe fines fraction, it is heat-treated at 1200° C. for 2 hours in anitrogen atmosphere. The heat-treated flake-shaped agglomerates arebroken by means of a sieve and sieving aid first to a size of less than3 mm, then to less than 700 μm. Finally the resultant flake-shapedagglomerates are broken by sieving to a size smaller than 350 μm andthen fractionated by further sieving to >210 μm and >100 μm, in order toseparate the fines fraction <100 μm of 30 wt. %.

Example 9

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5.2 kg/h as in example 1.

The granulated material obtained in the form of flakes with a thicknessof about 50 μm is then dedusted by sieving to <100 μm, with 28 wt. % ofthe material obtained in roll compaction being separated as finesfraction.

The separated coarse fraction is then heat-treated at 1200° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 5 wt. %.

Example 10

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5.2 kg/h as in example 1.

The granulated material obtained in the form of flakes with a thicknessof about 50 μm is then dedusted by sieving to <100 μm, with 26 wt. % ofthe material obtained in roll compaction being separated as finesfraction.

The separated coarse fraction is then heat-treated at 1600° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 4 wt. %.

Example 11

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5.2 kg/h as in example 1.

The granulated material obtained in the form of flakes with a thicknessof about 50 μm is then dedusted by sieving to <100 μm, with 26 wt. % ofthe material obtained in roll compaction being separated as finesfraction.

The separated coarse fraction is then heat-treated at 1950° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 2 wt. %.

Example 12

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5.2 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 8 kg/h. The granulated material obtained in the form offlakes with a thickness of about 50 to 100 μm is dedusted by sieving to<100 μm, with 16 wt. % of the material obtained in roll compaction beingseparated as fines fraction.

The separated coarse fraction is heat-treated at 1950° C. for 2 hours ina nitrogen atmosphere. The heat-treated flake-shaped agglomerates arebroken by means of a sieve and sieving aid first to a size of less than3 mm, then to less than 700 μm. Finally the resultant flake-shapedagglomerates are broken by sieving to a size smaller than 350 μm andthen fractionated by further sieving to >210 μm and >100 μm, in order toseparate the fines fraction <100 μm of 3 wt. %.

Example 13

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5.2 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 8 kg/h and for a third time with a throughput of 16 kg/h.The granulated material obtained in the form of flakes with a thicknessof about 50 to 200 μm is dedusted by sieving to <100 μm, with 5 wt. % ofthe material obtained in roll compaction being separated as finesfraction.

The separated coarse fraction is heat-treated at 1950° C. for 2 hours ina nitrogen atmosphere. The heat-treated flake-shaped agglomerates arebroken by means of a sieve and sieving aid first to a size of less than3 mm, then to less than 700 μm. Finally the resultant flake-shapedagglomerates are broken by sieving to a size smaller than 350 μm andthen fractionated by further sieving to >210 μm and >100 μm, in order toseparate the fines fraction <100 μm of 4 wt. %.

Example 14 I)

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 7.9 kg/h and for a third time with a throughput of 15.9kg/h. Between each compacting operation the fines fraction (<100 μm) wasdetermined, but not separated. The granulated material obtained in theform of flakes with a thickness of about 50 to 200 μm is then dedustedby sieving to <100 μm, with 12 wt. % of the material obtained in rollcompaction being separated as fines fraction.

The separated coarse fraction is then heat-treated at 1200° C. for 2hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving.

Example 14 I) A)

A <100 μm fraction is prepared from example 14 I) by sieving.

Example 14 I) B)

A 100-210 μm fraction is prepared from example 14 I) by sieving.

Example 14 I) C)

A 210-350 μm fraction is prepared from example 14 I) by sieving.

For examples 14I) A, B) and C), the values determined on theseagglomerate fractions for the texture index and the specific surfaceaccording to the BET method (nitrogen adsorption, Beckmann-Coulter) arepresented in Table 1,

and Table 1 also shows, for example 14 I) C), the aspect ratiodetermined from SEM photographs.

Example 14 II)

Example 14 I) was repeated, but the coarse fraction separated after rollcompaction three times was heat-treated at 1600° C. for 2 hours in anitrogen atmosphere.

Example 14 II) A)

A <100 μm fraction is prepared from example 14 II) by sieving.

Example 14 II) B)

A 100-210 μm fraction is prepared from example 14 II) by sieving.

Example 14 II) C)

A 210-350 μm fraction is prepared from example 14 II) by sieving.

For examples 14 II) A, B) and C), the values determined on theseagglomerate fractions for the texture index and the specific surfaceaccording to the BET method (nitrogen adsorption, Beckmann-Coulter) arepresented in Table 1, and Table 1 also shows, for example 14 II) C), theaspect ratio determined from SEM photographs.

Example 14 III)

Example 14 I) was repeated, but the coarse fraction separated after rollcompaction three times was heat-treated at 1950° C. for 2 hours in anitrogen atmosphere.

Example 14 III) A)

A <100 μm fraction is prepared from example 14 III) by sieving.

Example 14 III) B)

A 100-210 μm fraction is prepared from example 14 III) by sieving.

Example 14 III) C)

A 210-350 μm fraction is prepared from example 14 III) by sieving.

For example 14 III) C), the value determined on this agglomeratefraction for the texture index, the specific surface according to theBET method (nitrogen adsorption, Beckman-Coulter) and the aspect ratiodetermined from SEM photographs are presented in Table 1.

Example 15

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is granulated material in the form of flakes witha thickness of about 50 μm and a fines fraction of uncompacted startingmaterial and it is then dedusted by sieving to <100 μm, with 46 wt. % ofthe material obtained in roll compaction being separated as finesfraction.

The separated coarse fraction is then heat-treated at 1900° C. for 12hours in a nitrogen atmosphere. The heat-treated flake-shapedagglomerates are broken by means of a sieve and sieving aid first to asize of less than 3 mm, then to less than 700 μm. Finally the resultantflake-shaped agglomerates are broken by sieving to a size smaller than350 μm and then fractionated by further sieving to >210 μm and >100 μm,in order to separate the fines fraction <100 μm of 5 wt. %.

The specific surface was measured according to the BET method (nitrogenadsorption, Beckmann-Coulter) on the 210-350 μm fraction. The texturedagglomerates have a very low value of 0.99 m²/g. The texture indexmeasured on this fraction is 3.1.

Example 16

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 5 kg/h and for a third time with a throughput of 6.5 kg/h.The resultant material is granulated material in the form of flakes witha thickness of about 50 to 200 μm and it is then dedusted by sieving to<100 μm, with 15 wt. % of the material obtained in roll compaction beingseparated as fines fraction.

The separated coarse fraction is then broken by means of a sieve andsieving aid first to a size of less than 3 mm, then to less than 700 μm.Finally the resultant flake-shaped agglomerates are broken by sieving toa size smaller than 350 μm and then fractionated by further sieving to asize of 210-350 μm. The texture index determined on this agglomeratefraction is shown in Table 1.

Example 17

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5 kg/h as in example 1.

The resultant material is then compacted for a second time with athroughput of 7.9 kg/h and for a third time with a throughput of 15.9kg/h. The resultant material is granulated material in the form offlakes with a thickness of about 50 to 200 μm and it is then dedusted bysieving to <100 μm, with 15 wt. % of the material obtained in rollcompaction being separated as fines fraction.

The separated coarse fraction is then broken by means of a sieve andsieving aid first to a size of less than 3 mm, then to less than 700 μm.Finally the resultant flake-shaped agglomerates are broken by sieving toa size smaller than 350 μm and then fractionated by further sieving to asize of 210-350 μm. The texture index determined on this agglomeratefraction is shown in Table 1.

Example 18

BN—Al₂O₃, sprayed granules with an average granule size (d₅₀) of 100 μmand an Al content of 6 wt. % are fed continuously by means of avibratory chute and are roll-compacted with a throughput of 7 kg/h as inexample 1.

The resultant material is then compacted for a second time with athroughput of 12.9 kg/h and for a third time with a throughput of 19.9kg/h. The granulated material obtained in the form of flakes with athickness of about 50 to 200 μm is then deducted by sieving to <100 μm,with 22 wt. % of the material obtained in roll compaction beingseparated as fines fraction. These flakes have a diameter of severalcentimetres. They are then heat-treated in air at 1000° C. for one hourand can be used as filler for polymers.

Example 19

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h in a first pass as in example 1. In a secondpass (throughput 5 kg/h) and a third pass (throughput 6.5 kg/h),agglomerates were produced in the form of flakes with a thickness ofabout 50 to 200 μm and a diameter of several centimetres.

Small round disks (3.5 cm²) were cut out of the flakes obtained and atexture index of 11.4 was determined on these.

The density found both geometrically and by measurement of buoyancy isshown in Table 2.

Example 20

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 4 kg/h in a first pass as in example 1. In a secondpass (throughput 5 kg/h) and a third pass (throughput 6.5 kg/h),agglomerates were produced in the form of flakes with a thickness ofabout 50 to 200 μm and a diameter of several centimetres.

The agglomerates were heat-treated at 1950° C. for 2 hours undernitrogen.

Small round disks (3.5 cm²) were cut out of the flakes obtained and atexture index of 7.5 was determined on these.

The density found both geometrically and by measurement of buoyancy isshown in Table 2.

Example 21

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5 kg/h in a first pass as in example 1. In a secondpass (throughput 7.9 kg/h) and a third pass (throughput 15.9 kg/h)agglomerates were produced in the form of flakes with a thickness ofabout 50 to 200 μm and a diameter of several centimetres.

Small round disks (3.5 cm²) were cut out of the flakes obtained and atexture index of 14.4 was determined on these.

The density found both geometrically and by measurement of buoyancy isshown in Table 2.

Example 22

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μmis fed continuously by means of a vibratory chute and is roll-compactedwith a throughput of 5 kg/h in a first pass as in example 1. In a secondpass (throughput 7.9 kg/h) and a third pass (throughput 15.9 kg/h)agglomerates were produced in the form of flakes with a thickness ofabout 50 to 200 μm and a diameter of several centimetres.

The agglomerates were heat-treated at 1950° C. for 2 hours undernitrogen.

Small round disks (3.5 cm²) were cut out of the flakes obtained and atexture index of 66.9 was determined on these.

The density found both geometrically and by measurement of buoyancy isshown in Table 2.

Example 23

Example 22 was repeated, but the agglomerates were not heat-treated at1950° C., but at 2050° C. for 2 hours under nitrogen.

Small round disks (3.5 cm²) were cut out of the flakes obtained and atexture index of 108 was determined on these.

The density found both geometrically and by measurement of buoyancy isshown in Table 2.

In the following examples 24 to 32 and the comparative examples, boronnitride agglomerates were processed into filled epoxides (epoxide-boronnitride composites). Using these filled epoxide samples, comparativevalues can be obtained for the thermal conductivity of different boronnitride agglomerates. With other polymers, even higher thermalconductivity values can be reached, for example in the injectionmoulding of thermoplastics.

Example 24 I) A) Textured Agglomerates as Filler in Epoxide, Degree ofFilling 43%, Random Arrangement

53 g of textured agglomerates in flake form from example 7) I) A) aredispersed in 70 g of a mixture of epoxide and hardener (EpoFix, Struers,Willich, Germany) with a blade stirrer. At 0.2 bar, air is expelled in avacuum stirrer. The mixture is poured into dishes, in which itsolidifies.

To measure the thermal conductivity, a 10×10×2 mm³ plate is preparedfrom the middle of the moulding. The thermal conductivity TC isdetermined by measuring the quantities thermal diffusivity a, specificheat capacity c_(p) and density D and is calculated from thesequantities according to the equation TC=a*c_(p)*D. Measurement of a andc_(p) is performed with the Nanoflash LFA 447 (Netzsch, Selb, Germany)on samples with a size of 10×10×2 mm³ near room temperature. The densityis determined by weighing and determination of the geometric dimensionsof the precisely shaped samples. The measured value obtained for thethermal conductivity is shown in Table 3.

Examples 24I) B) and C), 24 II) A), B) and C), 24 III) A), B) and C),and Example 25 Textured Agglomerates as Filler in Epoxide, Degree ofFilling 43%, Random Arrangement

Example 24 I) A) was repeated, but using 53 g of textured agglomeratesin flake form according to the respective example stated in Table 3(“Filler” column).

Example 26 Textured Agglomerates as Filler in Epoxide, Degree of Filling43%, Arrangement with Preferred Orientation, through Plane

53 g of textured agglomerates in flake form from example 7) III) C) aredispersed in 70 g of a mixture of epoxide and hardener (EpoFix, Struers,Willich, Germany) with a blade stirrer. At 0.2 bar, air is expelled in avacuum stirrer. The mixture is cast between two acrylic glass disks,which are fixed with a distance of 2 mm between them. The mixturesolidifies between the acrylic glass disks. A sample with a size of10×10×2 mm³ is prepared from the solidified plate, the thickness of thesolidified plate being equal to the height of the sample.

The thermal conductivity is measured as described in example 24 I) A)(Table 3). Owing to the arrangement of the textured agglomerates withpreferred orientation, the thermal conductivity value determined ismainly “through plane” relative to the basal planes of the boron nitrideprimary particle lamellae.

Example 27 Textured Agglomerates as Filler in Epoxide, Degree of Filling43%, Arrangement with Preferred Orientation, in Plane

From the solidified plate from example 26, 5 lamellae with a size of10×10×2 mm³ are prepared. The lamellae are glued together, forming acube with 10 mm edge length. Now the cube is turned so that 5 end facesof the lamellae in the stack are facing upwards. The stack aligned inthis way is ground down from above and below to 2 mm thickness. Alamella with a size of 10×10×2 mm³ is formed again, which consists of 5segments glued together.

The thermal conductivity is measured as described in example 24 I) A)(Table 3). Owing to the arrangement of the textured agglomerates withpreferred orientation, the thermal conductivity value determined ismainly “in plane” relative to the basal planes of the boron nitrideprimary particle lamellae.

The in-plane value determined is higher than the through-plane valuefrom example 26.

Example 28 Textured Agglomerates as Filler in Epoxide, Degree of Filling43%, Arrangement with Preferred Orientation, through Plane

A mixture is prepared from 3% Boronid SCP1 (boron nitride powder withaverage particle size of 1 μm, ESK Ceramics GmbH & Co. KG), 24% BoronidS15 (boron nitride powder with average particle size of 15 μm, ESKCeramics) and 73% flakes from example 7 III) C). 53 g of the mixture isprocessed according to example 26, cast between two acrylic glass disks,producing a solidified plate. A thermal conductivity sample is preparedfrom the solidified plate as in example 26 and the thermal conductivityis measured (Table 3).

Example 29 Textured Agglomerates as Filler in Epoxide, Degree of Filling43%, Arrangement with Preferred Orientation, in Plane

Example 28 is repeated, but 5 lamellae with a size of 10×10×2 mm³ areprepared from the solidified plate. These are processed as in example 27into an in-plane thermal conductivity sample and the thermalconductivity in plane is measured (Table 3).

The in-plane value determined is higher than the through-plane valuefrom example 28.

Example 30

53 g of flakes from example 7 III) C) are treated mechanically in atumble mixer for 2 hours. The flakes are processed according to example24 I) A), a thermal conductivity sample is prepared and the thermalconductivity is measured (Table 3).

Example 31 Textured Agglomerates (not Heat-Treated) as Filler inEpoxide, Degree of Filling 59%

105 g of the 210-350 μm agglomerate fraction from example 17 isdispersed in 70 g of a mixture of epoxide and hardener (EpoFix, Struers,Willich, Germany) with a blade stirrer. At 0.2 bar, air is expelled in avacuum stirrer. The mixture is poured into dishes, where it solidifies.

For measurement of the thermal conductivity, a lamella with a size of10×10×2 mm³ is prepared from the middle of the moulding. The thermalconductivity is measured as described in example 24 I) A), and thethermal conductivity value determined is shown in Table 3.

Example 32 Textured Agglomerates (Heat-Treated) as Filler in Epoxide,Degree of Filling 59%

Example 31 was repeated, but 105 g of the agglomerates from example 14III) C) was processed instead of the agglomerates from example 32.

The thermal conductivity value determined (see Table 3) is higher thanin example 31.

Example 33 Hot Pressing with Textured Agglomerates

Hexagonal boron nitride powder with a primary particle size d₅₀ of 15 μm(measured using Mastersizer 2000, Malvern, wet measurement; from ESKCeramics GmbH & Co. KG, BORONID S15) is fed continuously by means of avibratory chute and is roll-compacted with a throughput of 5 kg/h as inexample 1. The resultant material is then compacted for a second timewith a throughput of 7.9 kg/h and for a third time with a throughput of15.9 kg/h. The granulated material obtained in the form of flakes with athickness of 200 μm is then dedusted by sieving to <100 μm, with 12 wt.% of the material obtained in roll compaction being separated as finesfraction. The flakes obtained have a size of 1-3 cm.

The flake-shaped agglomerates produced in this way were placed in ahot-pressing die (7.2 cm diameter, 12 cm height). Instead of the usual<250 g BN powder, more than 500 g of material could be filled in thesame volume of the hot-pressing die. Then hot-pressing was carried outat a temperature of 1800° C., a pressure of 23 MPa and with a holdingtime of one hour. The hot-pressed boron nitride sintered compactobtained has a density of 2.09 g/cm³ and has a very pronounced texture.The texture index determined on the hot-pressed sintered compact on asample prepared perpendicularly to the direction of hot pressing withthe dimensions 2 mm×1 cm×1 cm is 118.3. For comparison, the textureindex determined on known hot-pressed boron nitride sintered compacts(for example MYCROSINT S, ESK Ceramics GmbH & Co. KG) has values of upto about 1.8.

Example 34 Hot Pressing with Textured Agglomerates and TurbostraticBoron Nitride

As in example 33, textured agglomerates were produced, but hexagonalboron nitride powder with a primary particle size d₅₀ of 3 μm was usedfor roll compaction. The textured agglomerates with a size of 1-3 cmwere then mixed with turbostratic boron nitride in the ratio 1:1, filledin a hot-pressing die (7.2 cm diameter, 12 cm height) and hot-pressed ata temperature of 1800° C., a pressure of 23 MPa and a holding time of 90minutes. The hot-pressed boron nitride sintered compact obtained has adensity of 2.14 g/cm³. The 4-point-bending breaking strength determinedon this sintered compact is 153 MPa, and the Brinell hardness is 49.8.For comparison, a hot-pressed sintered compact of comparable size wasproduced from boron nitride powder with a primary particle size d₅₀ of 3μm in the same hot-pressing conditions. The bending breaking strengthdetermined on this comparative sintered compact is 75 MPa at a densityof 2.03 g/cm³ and a Brinell hardness of 31.8.

Example 35 Hot Pressing with Textured Agglomerates

Hexagonal boron nitride powder with a primary particle size d₅₀ of 3 μm(measured using Mastersizer 2000, Malvern, wet measurement) and a B₂O₃content of 3 wt. % is fed continuously by means of a vibratory chute andis roll-compacted with a throughput of 4 kg/h as in example 1. Theresultant material is then compacted for a second time with a throughputof 5.8 kg/h and for a third time with a throughput of 7 kg/h. Thegranulated material obtained in the form of flakes with a thickness of200 μm is then dedusted by sieving to <100 μm, with 4 wt. % of thematerial obtained in roll compaction being separated as fines fraction.The flakes obtained have a size of 1-3 cm.

The flake-shaped agglomerates produced in this way were placed in ahot-pressing die (7.2 cm diameter, 12 cm height). Then hot-pressing wascarried out at a temperature of 1800° C., a pressure of 23 MPa and witha holding time of one hour. The hot-pressed boron nitride sinteredcompact obtained has a density of 2.11 g/cm³. The texture index on thehot-pressed sintered compact on a sample prepared perpendicularly to thedirection of hot pressing with the dimensions 2 mm×1 cm×1 cm is 8.9.

Comparative Example 1 Untextured Agglomerates

Hot-pressed boron nitride (MYCROSINT S, ESK Ceramics GmbH & Co. KG) isprecrushed using a jaw crusher and then broken up in a hammer mill (with4 mm hole sieve insert). Sieving gives fractions in the desired range,i.e. 100-210 μm and 210-350 μm.

The value determined on the 100-210 μm agglomerate fraction for thetexture index and the specific surface according to the BET method(nitrogen adsorption, Beckmann-Coulter) are shown in Table 1.

Comparative Example 1 I) Untextured Agglomerates as Filler in Epoxide,Degree of Filling 43%, Random Arrangement

53 g of the agglomerates from comparative example 1 (fraction 100-210μm, with average agglomerate size d₅₀ of 170 μm) is processed accordingto example 24 I) A), a thermal conductivity sample is prepared and thethermal conductivity is measured (Table 3).

Comparative Example 1 II) Untextured Agglomerates as Filler in Epoxide,Degree of Filling 43%, Sample Preparation “through Plane”

53 g of the agglomerates from comparative example 1 (fraction 100-210μm, with average agglomerate size d₅₀ of 170 μm) is processed accordingto example 19 to a solidified plate, a thermal conductivity sample isprepared and the through-plane thermal conductivity is measured (Table3).

Comparative Example 1 III) Untextured Agglomerates as Filler in Epoxide,Degree of Filling 43%, Sample Preparation “in Plane”

53 g of the agglomerates from comparative example 1 (fraction 100-210μm, with average agglomerate size, d₅₀ of 170 μm) is processed accordingto example 19.

From the solidified plate from comparative example 1 II), 5 lamellaewith a size of 10×10×2 mm³ are prepared. These are processed as inexample 20 to an in-plane thermal conductivity sample and the thermalconductivity is measured in-plane (Table 3).

The in-plane value determined is only slightly higher than thethrough-plane value from comparative example 1 II).

Comparative Example 2 Untextured Agglomerates

Boron nitride agglomerates are produced according to example 3 of WO2005/021428 A1.

For this, hexagonal boron nitride powder with a primary particle sized₅₀ of 1 μm (measured using Mastersizer; from ESK Ceramics GmbH & Co.KG, BORONID SCP1) is compacted cold-isostatically at 1350 bar. Theresultant compacts are precrushed and broken up by means of sieving andsieving-trituration into a fraction of 100-210 μm. The granules areheat-treated under nitrogen at 1900° C. for 12 hours.

The resultant sinter cake of the granules is broken up again by sievingand sieving-trituration into a fraction of 100-210 μm. The averageagglomerate size d₅₀ measured by laser diffraction (dry measurement,Mastersizer 2000, Malvern) is 150 μm.

The value determined on the 100-210 μm agglomerate fraction for thetexture index and the specific surface according to the BET method(nitrogen adsorption, Beckmann-Coulter) are shown in Table 1.

Comparative Example 2 I) Untextured Agglomerates as Filler in Epoxide,Degree of Filling 43%, Random Arrangement

53 g of the agglomerates from comparative example 2 is processedaccording to example 17 I) A), a thermal conductivity sample is preparedand the thermal conductivity is measured (Table 3).

Comparative Example 2 II) Untextured Agglomerates as Filler in Epoxide,Degree of Filling 43%, Sample Preparation “through Plane”

53 g of the agglomerates from comparative example 2 is processedaccording to example, 26 to a solidified plate, a thermal conductivitysample is prepared and the through-plane thermal conductivity ismeasured (Table 3).

Comparative Example 2 III) Untextured Agglomerates as Filler in Epoxide,Degree of Filling 43%, Sample Preparation “in Plane”

From the solidified plate from comparative example 2 II), 5 lamellaewith a size of 10×10×2 mm³ are prepared. These are processed as inexample 27 to give an in-plane thermal conductivity sample and thethermal conductivity is measured in-plane (Table 3).

The in-plane value determined is only slightly higher than thethrough-plane value from comparative example 2 II).

Comparative Example 3

53 g of a powder mixture of 10% Boronid SCP1 (boron nitride powder withaverage particle size of 1 μm, ESK Ceramics GmbH & Co. KG) and 90%Boronid S15 (boron nitride powder with average particle size of 15 μm,ESK Ceramics GmbH & Co. KG) are processed as described in example 24 I)A), a thermal conductivity sample is prepared and the thermalconductivity is measured (Table 3).

TABLE 1 Example Texture index BET [m²/g] Aspect ratio  7 I) A) 1.6 16.651.8  7 I) B) 2.3 18.43 3.3  7 I) C) 2.9 18.63 n.d.  7 II) B) 2.5 8.773.3  7 II) C) 3.1 8.71 6.8  7 III) B) 3.4 7.54 3.5  7 III) C) 4.0 7.614.8 14 I) A) 7.5 6.35 n.d. 14 I) B) 16.4 7.19 n.d. 14 I) C) 17.6 8.494.4 14 II) A) 8.7 2.27 n.d. 14 II) B) 22.1 2.26 n.d. 14 II) C) 41.5 2.397.1 14 III) C) 40.5 1.69 7.6 15 3.1 0.99 n.d. 16 6.1 n.d. n.d. 17 7.7n.d. n.d. Comparative 1.4 2.79 n.d. example 1 Comparative 1.5 6.76 n.d.example 2 n.d.: not determined

TABLE 2 Density determined Buoyancy density geometrically Example[g/cm³] [g/cm³] 19 2.24 2.11 20 2.18 2.04 21 2.15 2.00 22 2.05 1.92 232.14 2.05

TABLE 3 Thermal conductivity Example Filler [W/m*K] 24 I) A)  7 I) A)1.06 24 I) B)  7 I) B) 1.35 24 I) C)  7 I) C) 1.03 24 II) A)  7 II) A)1.35 24 II) B)  7 II) B) 1.20 24 II) C)  7 II) C) 1.67 24 III) A)  7III) A) 3.15 24 III) B)  7 III) B) 2.13 24 III) C)  7 III) C) 2.26 25 14III) C) 1.90 26  7 III) C) 1.74 27  7 III) C) 2.59 28 3% SCP1, 24%Boronid 2.33 S15, 73% agglomerates from example 7 III) C) 29 3% SCP1,24% Boronid 3.19 S15, 73% agglomerates from example 7 III) C) 30  7 III)C) rounded 2.32 31 17 1.80 32 14 III) C) 2.40 Comparative Comparative1.26 example 1 I) example 1 Comparative Comparative 1.15 example 1 II)example 1 Comparative Comparative 1.24 example 1 III) example 1Comparative Comparative 2.26 example 2 I) example 2 ComparativeComparative 2.02 example 2 II) example 2 Comparative Comparative 2.17example 2 III) example 2 Comparative 10% SCP1, 90% Boronid 1.15 example3 S15

1. Boron nitride agglomerates, comprising lamellar, hexagonal boronnitride primary particles, which are agglomerated with one another witha preferred orientation, characterized in that the agglomerates formedare flake-shaped.
 2. Boron nitride agglomerates according to claim 1,characterized in that the boron nitride primary particles areagglomerated with one another with a preferred orientation in such a waythat the planes of the lamellae of the boron nitride primary particlesare essentially aligned parallel to one another.
 3. Boron nitrideagglomerates according to claim 1, characterized in that the textureindex measured on the flake-shaped agglomerates is more than 1.5,preferably at least 2.0, more preferably at least 2.8, especiallypreferably at least 3.5.
 4. Boron nitride agglomerates according toclaim 1, characterized in that the texture index measured on theflake-shaped agglomerates is 0.7 or less, preferably 0.5 or less, morepreferably 0.35 or less, especially preferably 0.3 or less.
 5. Boronnitride agglomerates according to claim 1, characterized in that thedensity of the flake-shaped agglomerates is at least 1.6 g/cm³,preferably at least 1.8 g/cm³, especially preferably at least 2.0 g/cm³.6. Boron nitride agglomerates according to claim 1, characterized inthat the proportion of forming surface, relative to the total surface ofthe flake-shaped agglomerates, is at least 10%, preferably at least 20%.7. Boron nitride agglomerates according to claim 1, characterized inthat the thickness of the flake-shaped agglomerates is 10 to 500 μm,preferably from 15 to 350 μm, especially preferably 30 to 200 μm. 8.Boron nitride agglomerates according to claim 1, characterized in thatthe average agglomerate size d₅₀ is up to 500 μm, preferably up to 200μm.
 9. Boron nitride agglomerates according to claim 1, characterized inthat, additionally to the boron nitride primary particles, theagglomerates comprise polymers and/or other particles, preferablyselected from the group comprising carbides, borides, nitrides, oxides,hydroxides, carbon and metals.
 10. Boron nitride agglomerates accordingto claim 1, characterized in that the agglomerates are surface modifiedby means of additives, and in that the additives are preferably selectedfrom the group comprising polymers, organometallic compounds, silanes,oils, carboxylic acids, copolymers, hydrolysed monomers, partiallyhydrolysed monomers, partially condensed monomers and nanoparticles. 11.Method for producing boron nitride agglomerates according to claim 1,characterized in that lamellar, hexagonal boron nitride primaryparticles are agglomerated in such a way that they line up with oneanother with a preferred orientation.
 12. Method for producing boronnitride agglomerates according to claim 11, characterized in that theagglomeration of the boron nitride primary particles is effected by thecompaction of a starting material comprising lamellar, hexagonal boronnitride between two counter rotating rolls.
 13. Method for producingboron nitride agglomerates according to claim 12, characterized in thatthe rolls are pressed against each other with a defined contactpressure.
 14. Method for producing boron nitride agglomerates accordingto claim 12, characterized in that the compaction step is carried outseveral times.
 15. Method for producing boron nitride agglomeratesaccording to claim 12, characterized in that after compaction, a thermaltreatment is carried out as a further step, preferably at a temperatureof up to 2300° C., more preferably at 1200 to 2050° C., still morepreferably at 1400 to 2000° C. and especially preferably at 1600 to1950° C.
 16. Method for producing boron nitride agglomerates accordingto claim 11, characterized in that following the agglomeration step, adefined agglomerate size fraction is obtained by classifying, saidclassifying being effected by methods known in the prior art, preferablyby sieving and screening.
 17. Polymer-boron nitride composite comprisingflake-shaped boron nitride agglomerates according to claim
 1. 18. Boronnitride sintered compact, comprising hexagonal boron nitride, obtainableby sintering of flake-shaped boron nitride agglomerates according toclaim
 1. 19. Boron nitride sintered compact according to claim 18,characterized in that the texture index of the hexagonal boron nitridein the sintered compact is at least 2, preferably at least 3, morepreferably at least 5, more preferably at least 10 and especiallypreferably at least 20.